­Exploding lakes are rare, and the backstories of Lakes Nyos and Monoun explain why. In Cameroon, there are weak spots in the Earth's crust at which magma, or liquid rock, rises from the Earth's mantle. The magma shoots up quickly and vertically, cutting a tube toward the surface. If it reaches the surface, the magma may spurt out and rain a big pile of rock, depositing a cinder cone volcano.

If the magma hits wet rock as it rises, it explodes, blasting a crater in the ground. More than 18,000 years ago, such a blast formed the crater at Lake Monoun [source: Sigurdsson]. A si­milar blast happened a few hundred years ago to form Nyos [source: ­Kling]. Water filled the craters, and they became lakes.

At the bottom of each lake, the old tube where magma first rose to the surface remains. If you follow the tube some 3 to 6 miles (5 to 10 kilometers) down, you'll hit magma. The pressure down there forces out one of the most abundant gases in liquid rock: CO2. The gas rises up the tube into the lake. Researchers have identified more than 100 places in Cameroon where CO2 leaks in large, but not dangerous quantities out of the ground, says Evans.

Several factors -- not just CO2 -- have to align to create an exploding lake. First, the lake must be deep. When little water holds down the gaseous bottom water, the lake needs only a small disturbance -- wind -- to release the gas. In deep lakes, the overlaying water acts as a cork in a champagne bottle. Every 10 meters (33 feet) of water adds 1 atmosphere of pressure, so in a 100-meter (328-foot) lake, 10 atmospheres of pressure hold down the gas at the bottom, says Evans. Wind can't stir it up.

Second, the climate must be stable all year, which is why exploding lakes cluster in the tropics. Lake Superior in the United States, for instance, charges with gas from decaying matter until the season changes. Every fall, the lake's surface cools and gets denser, then sinks to the bottom. The gaseous bottom water rises. The lake turns over, or exhales -- most lakes do at least once a year, says Varekamp. In places where it's warm or cold year-round, lake layers hold their temperature and position. Third, it takes a trigger like a landslide, earthquake or too much gas to unsettle the gas layer.

Cameroon has all the right ingredients for exploding lakes: magma releasing CO2­ into deep lakes, a tropical climate and a trigger.